.tip {
  position: fixed;
  top: 0;
  left: 0;
  right: 0;
  bottom: 0;
  background-color: rgba(0, 0, 0, 0.6);
}
.box {
  width: 400px;
  height: 400px;
  background-color: #fff;
  margin: 50px auto;
  display: inline-block;
}